1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus for driving a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enhancing a picture quality.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a dynamic image. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal, such as a viscosity and an elasticity, etc. Such characteristics can be explained by using the following equations (1) and (2):τr∝γd2/Δε|Va2−VF2|  (1)where τr represents a rising time when a voltage is applied to a liquid crystal, Va is an applied voltage, VF represents a Freederick transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of the liquid crystal cells, and γ represents a rotational viscosity of the liquid crystal molecules.τf=γd2/K  (2)where τf represents a falling time at which a liquid crystal is returned into the initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an elastic constant.
A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of the liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage charged in the liquid crystal cell is progressed into the next frame prior to arriving at a target voltage. Thus, due to a motion-blurring phenomenon a screen is blurred out at the moving picture.
Referring to FIG. 1, the conventional LCD cannot express desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested to modulate data in accordance with a difference in the data by using a look-up table (hereinafter referred to as high-speed driving method). This high-speed driving method allows data to be modulated by a principle as shown in FIG. 2.
Referring to FIG. 2, a conventional high-speed driving method modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. This high-speed driving method increases |Va2−VF2| from the above equation (1) on the basis of a difference in the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving method compensates for a slow response time of the liquid crystal by modulating a data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at desired color and brightness.
In other words, the high-speed driving method detects a variation in most significant bit data through a comparison of most significant bit data MSB of a current frame Fn with most significant bit data MSB of the previous frame Fn−1. If the variation in the most significant bit data MSB is detected, a modulated data corresponding to the variation is selected from a look-up table so that the most significant bit data MSB is modulated as shown in FIG. 3. The high-speed driving method modulates only a part of the most significant bits among the input data for reducing a memory capacity. For example, the high-speed driving method can be implemented as shown in FIG. 4.
Referring to FIG. 4, a conventional high-speed driving apparatus includes a frame memory 43 connected to a most significant bit output bus line 42 and a look-up table 44 connected to the most significant bit output bus line 42 and an output terminal of the frame memory 43.
The frame memory 43 stores most significant bit data MSB during one frame period and supplies the stored data to the look-up table 44. Herein, the most significant bit data MSB are high-order 4 bits among 8 bits of the source data RGB.
The look-up table 44 makes a mapping of the most significant bit data of the current frame Fn inputted from the most significant bit output bus line 42 and the most significant bit data of the previous frame Fn−1 inputted from the frame memory 43 into a modulation data table such as Table 1 to select modulated most significant bit data Mdata. Such modulated most significant bit data Mdata are added to an non-modulated least significant bit data LSB from a least significant bit output bus line 41 before outputting to a liquid crystal display.
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
In the above Table 1, a left column is for a data voltage VDn−1 of the previous frame Fn−1 while an uppermost row is for a voltage VDn of the current frame Fn.
Such a conventional high-speed driving method enhances a dynamic contrast ratio in comparison with a conventional normal driving method that does not modulate the source data. However, the conventional high-speed driving method gradually enhances brightness so that a desired brightness level is achieved at the end of one frame period. Due to this, in the conventional high-speed driving method, the dynamic contrast ratio cannot be reached at a desired level. Furthermore, a color represented by combining red, green, and blue is distorted when those colors are reproduced.